Nuclear fuel assembly

ABSTRACT

The present invention relates to a fuel assembly with a substantially square cross section for a light-water reactor. The light-water reactor comprises a plurality of fuel rods ( 4 ) extending between a top tie plate ( 5 ) and a bottom tie plate ( 6 ). A fuel rod ( 4 ) comprises a cladding tube ( 7 a) with a first and a second end which surround a column with fissionable material ( 7 b). According to one aspect of the invention, at least one fuel rod ( 4 ) is provided with an axial gap ( 19 ) in the fissionable material ( 7 b), such that fissionable material ( 7 b) is arranged on both sides of the axial gap ( 19 ) in the fuel rod ( 4 ).

TECHNICAL FIELD

[0001] The present invention relates to a nuclear fuel assembly with asubstantially square cross section for a light water reactor comprisinga plurality of fuel rods extending between a top tie plate and a bottomtie plate.

BACKGROUND ART

[0002] In a nuclear reactor, moderated by means of light water, the fuelexists in the form of fuel rods, each of which contains a stack ofpellets of a nuclear fuel arranged in a cladding tube, a column ofextruded fuel cylinders or an uninterrupted column ofvibration-compacted powdered fuel. The cladding tube is normally made ofa zirconium-base alloy. A fuel bundle comprises a plurality of fuel rodsarranged in parallel with each other in a certain definite, normallysymmetrical pattern, a so-called lattice. The fuel rods are retained atthe top by a top tie plate and at the bottom by a bottom tie plate. Tokeep the fuel rods at a distance from each other and prevent them frombending or vibrating when the reactor is in operation, a plurality ofspacers are distributed along the fuel bundle in the longitudinaldirection. A fuel assembly comprises one or more fuel bundles, each oneextending along the main part of the length of the fuel assembly.

[0003] Together with a plurality of other fuel assemblies, the fuelassembly is arranged in a core. The core is immersed in water whichserves both as coolant and as neutron moderator. During operation, thewater flows from below and upwards through the fuel assembly, whereby,in a boiling water light-water reactor, part of the water is transformedinto steam. The percentage of steam increases towards the top of thefuel assembly. Consequently, the coolant in the lower part of the fuelassembly consists of water whereas the coolant in the upper part of thefuel assembly consists both of steam and of water. This differencebetween the upper and lower parts gives rise to special problems whichmust be taken into consideration when designing the fuel assembly.

[0004] This problem can be solved by achieving a flexible fuel assemblywhich, in a simple manner, may be given a shape in which the upper partof the fuel assembly differs from the lower part thereof such thatoptimum conditions can be obtained. A fuel assembly for a boiling waterreactor with these properties is shown in PCT/SE95/01478 (Int. Publ. No.WO 96/20483). This fuel assembly comprises a plurality of fuel unitsstacked on top of each other, each comprising a plurality of fuel rodsextending between a top tie plate and a bottom tie plate. The fuel unitsare surrounded by a common fuel channel with a substantially squarecross section. A fuel assembly of this type may in a simple manner begiven a different design in its upper and lower parts.

[0005] Also in a light-water reactor of pressurized-water type, it maybe desirable to design the fuel assemblies such that each fuel assemblycomprises a plurality of fuel units stacked on top of each other. Asdescribed above, each one of the fuel units then comprises a pluralityof fuel rods extending between a top nozzle and a bottom nozzle. A fuelassembly for a pressurized-water reactor, however, comprises no fuelchannel.

[0006] One factor which must be taken into consideration when designingsuch fuel units with a length of the order of size of 300-1500millimeters is that fission gases are formed during nuclear fission. Inaddition, the column of fuel pellets expands because of the heatgenerated in the fuel pellets. To take care of the fission gases and thethermal expansion of the column of fuel pellets, a relatively largespace, an axial gap, is normally formed above the uppermost fuel pelletin the cladding tube in known full-length fuel rods, that is, fuel rodswith a length of the order of size of 4 meters. The axial gap has alength of the order of size of 200-300. The fission gases may thusdiffuse to this axial gap and the column of fuel pellets may expand intothis gap.

[0007] Another factor which must be taken into consideration whendesigning axial gaps is that the temperature of the cladding tube inthis region is lower than in the rest of the cladding tube since no fuelpellet is arranged in the axial gap. A problem which may arise as aresult of this is that hydrogen formed, inter alia, by corrosion of thecladding tube, which is of a zirconium-based alloy, and is taken upthereby, diffuses into this colder region. In the event that theconcentration of hydrogen becomes too high in this region, hydrides areformed in the cladding material and cause embrittlement thereof. In aserious case, the cladding tube may burst and fissionable material mayenter into the cooling water. The same type of problem may also arise inthe regions between the pellets, that is, where a lower end of a fuelpellet makes contact with an upper end of an adjacent fuel pellet, andin the region between two fuel units stacked on top of each other. Therisk of embrittlement due to too high a concentration of hydrogenincreases, to a certain limit, with the size of the axial gap.

[0008] Released fission gas contributes to the temperature in the axialgap decreasing further. This is due to the fission gas deteriorating thethermal conductivity of the gas which is present in the axial gap. Thesame thing applies to the gas which is present in the gap between thefuel pellets and the cladding tube, in which case the difference intemperature between the outer surface of the pellets and the innersurface of the cladding tube increases.

[0009] It is known to reduce the release of fission gas in differentways. One such way is to provide one or more of the fuel pellets withthrough-holes in their axial directions. In this way, the temperature inthe fuel pellet is lowered whereby the release of fission gas is reducedand the axial gap may be reduced. In this case, the axial gap may belimited to the order of size of a few millimeters in a rod with a lengthof the order of size of 300 millimeters, up to a few tens of millimetersfor longer rods, to allow the thermal expansion of the column of fuelpellets. A disadvantage of pellets provided with through-holes is thatthey are complicated to manufacture. For that reason, it is desirable toarrange axial gaps in the fissionable material.

[0010] Still another factor which must be taken into consideration whendesigning axial gaps in a fuel rod is that local power peaks arise here.The power peaks arise due to the moderation in this region, wherefissionable and neutron-absorbing material are missing, being very good.This results in the power in the pellets adjoining the axial gapbecoming very high, that is, a power peak arises. The power peak growswith the size of the axial gap.

[0011] The object of the present invention is to provide a fuel assemblywith a plurality of short fuel units with fuel rods formed with axialgaps in the fissionable material adapted to give rise to small powerpeaks only.

SUMMARY OF THE INVENTION

[0012] The present invention relates to a fuel assembly comprising aplurality of fuel rods, each having at least one axial gap for fissiongases, formed during operation, and thermal expansion of the nuclearfuel. The features which characterize this fuel assembly are stated inclaim 1.

[0013] The fuel assembly comprises a cladding and a stack of nuclearfuel pellets arranged therein. The cladding tube is sealed with a plugat each end, more particularly with a top plug and a bottom plug. Theaxial gaps in the fuel rods are arranged such that, in adjacentlyarranged fuel rods, they are disposed at axially separated levels. Byavoiding to arrange axial gaps at the same levels in adjacently arrangedfuel rods, the risk of high power peaks is reduced as a consequence ofthe good moderation in this region.

[0014] To further reduce the power peaks at the axial gaps, in oneembodiment of the invention these gaps are distributed at a plurality oflevels within one and the same fuel rod. In this way, each one of theaxial gaps may be made considerably smaller than if only one gap isarranged in the fuel rod.

[0015] To achieve the axial gaps at the desired level in the fuel rod, aspacer is arranged in the axial gap or gaps. The spacer is designeddeformable in the axial direction. In this way, the column of fuelpellets is allowed, because of thermal expansion, to be extended intothe axial gap or gaps while the spacer is being deformed. When thespacer has been deformed in the axial direction, it prevents, byfriction against the wall of the cladding tube, axial gaps from arisingin the upper part of the fuel rod also when the fuel pellets decrease insize because of densification. Alternatively, the spacer may be designedresilient, for example in the form of a spiral spring with the samefunction as described above.

[0016] By not arranging the axial gaps in traditional manner, that is,above or below the column with the fissionable material in the fuelrods, the power peaks between two fuel units stacked on top of eachother are reduced. The axial gaps are achieved by arranging a spacer atan arbitrary level in the column of fissionable material. To furtherreduce the power peaks in the upper and lower ends, respectively, of thefuel rods, that is, between two fuel units stacked on top of each other,the fuel pellets in these regions may be designed with a smallerdiameter than the other fuel pellets. To avoid annular gaps between thefuel pellet and the cladding tube, that part of the fuel rod whichsurrounds the fuel pellet and the cladding tube is designed with acorrespondingly smaller inner diameter which has the same extent in theaxial direction as the fuel pellet. Alternatively, the fuel pellets inthis region may be given a lower enrichment.

[0017] The advantage of the invention is that axial gaps comprising thespacers which may be placed in optional positions are avoided in theupper parts of the fuel rods. The region without fissionable materialformed between two fuel units stacked on top of each other is thusreduced and hence also the local power peak which may arise in thisregion due to too good moderation.

[0018] Another advantage is that the necessary axial gap, by means ofthe spacers which may be located in optional positions, may be dividedinto a plurality of smaller axial gaps whereby the power peaks thereinare reduced. At the same time, the risk of too high a concentration ofhydrogen in the axial gaps is reduced.

[0019] At least to a certain extent, the spacer contributes to increasethe temperature somewhat in the material surrounding the axial gap incomparison with the temperature of axial gaps without spacers. Theincreased temperature is due to the spacer conducting part of the heat,which is generated in the pellets facing the axial gap, to the claddingtube. By this increased temperature, the risk of the hydrogenconcentration becoming too high in the axial gaps is further reduced.

[0020] Still another advantage is that the spacer, even at the time ofmanufacture of the fuel rods, may accumulate a certain length toleranceof the pellets column. This means that the requirement for the lengthtolerance of the individual fuel pellets is reduced.

BRIEF DESCRIPTION OF THE DRAWINGS

[0021]FIG. 1a shows in a vertical section a fuel assembly of a boilingwater type with short fuel units.

[0022]FIG. 2 shows a section A-A of the fuel assembly in FIG.

[0023]FIGS. 2a and 2 b show alternative embodiments of a fuel assemblyof the same type as that shown in FIG. 1 in a section corresponding tothe section A-A of the fuel assembly in FIG. 1.

[0024]FIG. 3 shows in a vertical section a fuel assembly ofpressurized-water type with short fuel units.

[0025]FIG. 4 shows a fuel rod for a fuel unit according to FIG. 1 or 2with a spacer arranged in an axial gap.

[0026]FIG. 5a shows two adjacently located fuel rods, each with an axialgap, wherein the axial gaps are arranged at axially separate levels.

[0027]FIG. 5b shows two adjacently located fuel rods, each with twoaxial gaps, wherein all the axial gaps are arranged at axially separatelevels.

[0028]FIG. 6a shows a fuel rod with a plurality of axial gapsdistributed along their axial length.

[0029]FIG. 6b shows a detail of FIG. 6a, wherein a spacer is arranged inan axial gap.

[0030]FIG. 7a shows a spacer in a view from the side.

[0031]FIG. 7b shows the spacer according to FIG. 7a in a view fromabove.

[0032]FIG. 8 shows a fuel rod with an upper and a lower end pellet witha smaller diameter than the other fuel pellets and a top plug and abottom plug, respectively, which are intended to surround the end plugs.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0033]FIG. 1 shows a fuel assembly of a boiling water type comprising anupper handle 1, a lower end portion 2 and a plurality of fuel units 3stacked one above the other. Each fuel unit 3 comprises a plurality offuel rods 4 arranged in parallel and in spaced relationship to eachother in a given lattice. Further, each fuel unit 3 comprises a top tieplate 5 and a bottom tie plate 6 for attachment of the fuel rods 4 intheir respective positions in the lattice. The fuel units 3 are stackedon top of each other in the longitudinal direction of the fuel assemblyand they are stacked in such a way that the top tie plate 5 in one fuelunit 3 is facing the bottom tie plate 6 in the next fuel unit 3 in thestack and such that the fuel rods 4 in all the fuel units 3 are parallelto one another. A fuel rod 4 contains fuel in the form of a stack offuel pellets 7 b of uranium arranged in a cladding tube 7 a. Thecladding tube 7 a is suitably made of a zirconium-base alloy or an alloywhich, in addition to zirconium, comprises niobium, iron, tin andchromium. A coolant is adapted to flow from below and up through thefuel assembly.

[0034]FIG. 2 shows that the fuel assembly is enclosed in a fuel channel8 with a substantially square cross section. The fuel channel 8 isprovided with a hollow support member 9 of cruciform cross section,which is secured to the four walls of the fuel channel 8. In the centralchannel 14 formed of the support member 9, moderator water flows. Thefuel channel with support members surround four vertical channel-formedparts 10, so-called sub-channels, with an at least substantially squarecross section. The four sub-channels each comprises a stack of fuelunits 3. Each fuel unit 3 comprises 24 fuel rods 4 arranged in asymmetrical 5×5 lattice.

[0035] The fuel assembly in FIG. 2 comprises 10×10 fuel rod positions.By a fuel rod position is meant a position in the lattice. All the fuelrod positions in the lattice need not be occupied by fuel rods 4. Incertain fuel assemblies, a number of fuel rods 4 are replaced by one ora plurality of water channels. The introduction of a water channelchanges the number of fuel rods 4 but not the number of fuel rodpositions.

[0036]FIG. 2a shows an alternative embodiment of a fuel assemblyaccording to the invention. FIG. 2a shows a horizontal section throughthe fuel assembly which is provided with an internally arranged verticalchannel 14 a through which water is conducted in a vertical directionfrom below and upwards through the fuel assembly. The channel 14 a issurrounded by a tube 9 a with a substantially square cross section. Thefuel units 3 are kept in position by being fitted onto the tube whichsurrounds the vertical channel 14 a.

[0037]FIG. 2b shows an additional embodiment of a fuel assemblyaccording to the invention. The figure shows a horizontal sectionthrough the fuel assembly which is provided with two centrally arrangedvertical water rods 14 b through which water is conducted from below andupwards through the fuel assembly. The water rods 14 b have a diameterwhich is somewhat larger than the diameter of the fuel rods 4 and areformed with a substantially circular cross section. The fuel units 3 arekept in position by being fitted onto the water rods 14 b.

[0038]FIG. 3 shows a pressurized-water fuel assembly of square crosssection. In the same way as the fuel assembly in FIG. 1, it comprises aplurality of fuel units 3 stacked on top of each other. Each fuel unit 3comprises a plurality of fuel rods 4 arranged in parallel and in spacedrelationship to each other in a given lattice. Each fuel unit 3 furthercomprises a top tie plate 5 and a bottom tie plate 6 for attachment ofthe fuel rods 4 in their respective positions in the lattice. The fuelunits 3 are stacked on top of each other in the longitudinal directionof the fuel assembly and they are stacked in such a way that the top tieplate 5 in one fuel unit 3 is facing the bottom tie plate 6 in the nextfuel unit 3 in the stack, and such that the fuel rods 4 in all the fuelelements 3 are parallel to each other. A fuel rod 4 contains fissionablematerial in the form of a stack of fuel pellets 7 b of uranium arrangedin a cladding tube 7 a. A coolant is adapted to flow from below andupwards through the fuel assembly. A number of so-called control rodguide tubes 4 b are arranged extending through the whole fuel assembly.The control rod guide tubes 4 b are intended to receive finger-shapedcontrol rods (not shown) which are inserted into and withdrawn from,respectively, the guide tubes 4 a for the purpose of controlling thepower of the nuclear reactor. The guide tubes extend between a top part15 and a bottom part 16. The top part 15 is arranged above the uppermostfuel unit 3 in the fuel assembly and the bottom part 16 is arrangedbelow the lowermost fuel unit 3 in the fuel assembly. The fuel units 3are kept in position by being fitted onto the control rod guide tubes 4b.

[0039]FIG. 4 shows a fuel rod 4 for a fuel assembly according to FIG. 1or FIG. 3. The fuel rod 4 comprises, as mentioned above, a cladding tube7 a and a stack of fuel pellets 7 b arranged in the cladding tube. Atthe top, the cladding tube 7 a is sealed with a top plug 17 and at thebottom with a bottom plug 18. The fuel rod 4 is formed with an innercavity, an axial gap 19, in which fission gases may accumulate. Theaxial gap 19 is also intended to permit thermal expansion of the columnof fuel pellets 7 a.

[0040] A spacer 20 made of a zirconium-base alloy is arranged in thecolumn of fuel pellets 7 b to achieve the axial gap 19 at the desiredlevel in the fuel rod. The axial gap 19 is arranged such that at leastone fuel pellet 7 b is arranged between the axial gap and either the topplug 17 or the bottom plug 18 of the fuel rod 4. The spacer 20 is formedas a sleeve with V-shaped slits 21 arranged in the respective ends. Theouter parts of the tongues 22 formed between the slits are bent intowards the centre of the spacer 20 at an angle of the order ofmagnitude of 100°. The spacer 20 is adapted to make contact, by itsupper end, with a lower end of a fuel pellet 7 b and, by its lower end,to make contact with an upper end of a fuel pellet 7 b. This design ofthe spacer 20 permits the spacer to be deformed in the axial directionwhen the fuel pellets 7 b because of thermal expansion grow in the axialdirection. When the spacer 20 is deformed, it will make contact with theinner surface of the cladding tube 7 a. This means that the pelletscolumn across such a spacer 20, because of its friction against thecladding tube, also when the pellets 7 b shrink due to densification, isretained in its position. In this way, axial gaps 19 are prevented fromforming between the top plug 17 and the fuel pellet 7 b arranged at thetop of the column.

[0041] The spacer 20 may, of course, be formed in many different ways.It may, for example, be provided with an edge, folded towards thecentre, without slits 21. Alternatively, it may be formed as a spiralspring. It may also be suitable to arrange different types of spacers 20in different parts of the fuel rod 4, for example non-deformable spacers20 in certain axial gaps 15 a.

[0042] In FIG. 4, it is indicated that the pellet 7 b arranged at thetop and bottom of the fuel rod 4, as well as the pellets 7 b arrangedadjacent the spacer 20, are made with through-holes 23. With thisembodiment, the maximum temperature in the fuel pellets 7 b may bereduced in the region where power peaks due to good moderation arise. Atthe same time, the amount of released fission gas may be reduced andspace for accumulation of released fission gases be created in thepellets 7 b. Further, the fuel pellets 7 b are provided with cuppedupper and lower end surfaces (see reference numeral 24). Because of thethermal expansion, the fuel pellets 7 b grow more in the central, warmerparts than in the outer, colder parts. The cup shape 24 thus permitsthermal expansion to a certain extent before the axial gap 19 isutilized for this purpose. Because of the hollowed 23 and cup-shaped 24pellets 7 b, a smaller axial gap 19 is sufficient for the thermalexpansion and for accumulation of the released fission gases.

[0043] Alternatively, fuel pellets 7 b with lower enrichment may be usedadjacent the spacers 20. This has, in principle, the same effect ashollowed pellets when it comes to limiting power peaks, however, notwith regard to reducing the power at the centre of the fissionablematerial or accumulating fission gases.

[0044]FIG. 5a shows two fuel rods 4 arranged adjacent to each other,each with an axial gap 19. The axial gaps 19 in the two adjacentlyarranged fuel rods 4 are arranged at axially separate levels. Arrangingaxial gaps 19 at axially separate levels in adjacently located fuel rods4 results in an equalization of the power along the fuel rod 4 and areduced risk of high power peaks as a result of too good moderation inthese regions which lack fissionable material 7 b.

[0045] In an alternative embodiment, an axial gap 19 is arranged atrandom in the fuel rod 4 during the manufacture. It is then suitable todetermine in advance a region within which the location of the axial gap19 may be varied. The random location of the axial gap 19 may, forexample, be achieved with the aid of a conventional random numbergenerator.

[0046]FIG. 5b shows an alternative embodiment of the fuel rod 4according to FIG. 5a. The axial gap 19 is here divided into two smalleraxial gaps 19 a in each fuel rod 4. The axial gaps 19 a are arranged atdifferent axial levels in the respective fuel rods 4.

[0047] The fuel rods 4 in FIG. 5a and FIG. 5b are designed preferablyidentical, but when putting these together into a bundle for a fuelassembly, every other fuel rod 4 is placed upside down.

[0048]FIG. 6a shows another alternative embodiment of the fuel rod 4. Inthis fuel rod 4, the axial gap 19 is divided into four smaller axialgaps 19 b arranged at axially separate levels. In this case, fuelpellets 7 b without through-holes 23 may be used. In FIG. 6b, a spacer20 is shown which is adapted to such a short axial gap 19 b.

[0049] A fuel assembly which has a length of the order of size of 400millimeters is provided with a gap which is 20-30 millimeters,alternatively two gaps which are each of the order of size of 10millimeters, etc.

[0050]FIGS. 7a and 7 b show an alternative embodiment of the spacer 20.This spacer 20 is formed by punching a sheet and forming the sheet intoa sleeve and folding the tongues 22 inwards towards the centre of thesleeve. The dash-lined slit 25 shown in the figure indicates where theends of the sheet meet. By forming the slit 25 with a hook 25 a, thespacer 20 may be given a stable design in the axial direction. Thisspacer 20 is simple to manufacture since it is punched out in a piece ofsheet, whereafter it is formed into a sleeve.

[0051]FIG. 8 shows an embodiment of a fuel rod 4 with an upper and alower end pellet 7 c with a smaller diameter than that of the other fuelpellets 7 b. Because of this arrangement, the power peaks at the axialgap between fissionable material 7 b which is formed between two fuelunits stacked on top of each other may-be reduced (see reference numeral19 c in FIGS. 1 and 3, respectively). To prevent gaps from arisingbetween the fuel pellet 7 c and the top plug 17 and the bottom plug 18,respectively, the material surrounding the end pellet 7 c is made with acorrespondingly smaller inner diameter. In FIG. 8, the top plug 17 andthe bottom plug 18, respectively, are provided with a larger thicknessof material in relation to the cladding tube 7 a. The material aroundthe end pellet 7 c may, of course, be provided with a correspondinglysmaller inner diameter in some other manner than with the aid of the topplug 17 and the bottom plug 18, respectively; for example, the claddingtube 7 a itself may be designed in this way.

1. A fuel assembly for a light-water reactor with a substantially squarecross section, characterized in that it comprises a plurality of shortfuel units (3), each comprising a plurality of fuel rods (4) extendingbetween a top tie plate (5) and a bottom tie plate (6), wherein a fuelrod (4) comprises a cladding tube (7 a) with a first and a second end,wherein the cladding tube (7 a) surrounds a column of fissionablematerial (7 b), and that at least one fuel rod (4) is provided with anaxial gap (19) in the fissionable material (7 b) and that fissionablematerial (7 b) is arranged on both sides of the axial gap (19) in thefuel rod (4).
 2. A fuel assembly according to claim 1 , characterized inthat the axial gap (19) is divided into two or more smaller axial gaps(19 a, 19 b) and that fissionable material (7 b) is arranged between theaxial gaps (19 a, 19 b).
 3. A fuel assembly according to claim 1 or 2 ,characterized in that the axial gap or gaps (19) in adjacently locatedfuel rods (4) are arranged at axially separate levels.
 4. A fuelassembly according to any of the preceding claims, characterized in thatthe axial gap or gaps (19, 19 a, 19 b) comprise a deformable ornon-deformable spacer (20), which is resilient in the axial direction,for separating the fissionable material (7 b) which is arranged aboveand below the spacer.
 5. A fuel assembly according to claim 4 ,characterized in that the spacer (20) is formed as a sleeve and that thesleeve at each end is provided with V-shaped slits (21) and that thetongues (22), formed between the slits, are folded inwards towards thecentral part of the spacer (20) at an angle of the order of magnitude of100°.
 6. A fuel assembly according to claim 4 or 5 , characterized inthat the spacer (20) is adapted to make contact with the inner surfaceof the cladding tube (7 a) and, by friction against the cladding tube,to fix the fissionable material (7 b) arranged thereabove in the axialdirection.
 7. A fuel assembly according to claim 4 , 5 or 6,characterized in that the spacer (20) is made of a zirconium-basedalloy.
 8. A fuel assembly according to any of the preceding claims,characterized in that fissionable material (7 c) with a smaller diameterthan the main part of the fissionable material (7 b) is arranged at thefirst and/or second end of the cladding tube (7 a) and that the claddingtube (7 a) at this end is provided with a correspondingly smaller innerdiameter.
 9. A fuel assembly according to any of the preceding claims,characterized in that the fissionable material (7 b) is formed aspellets of nuclear fuel and that the pellets which are arranged close toan axial gap (19, 19 a, 19 b) and/or at the first or second end of thecladding tube are provided with through-holes (23) or with lowerenrichment.